BLUE SUN: Spaceweather.com reports that for the second time this year, a blue sun has appeared over Egypt. "A powerful khamaseen dust storm swept through Alexandria on March 7th," reports astronomer Aymen Ibrahem of the Bibliotheca Alexandrina. "The sun shining through the dust turned blue."

The Egyptian sun on March 7th. Photo credit: Aymen Ibrahem.

Blue suns occur when the air fills with particles just a little larger than the wavelength of light. This makes the air behave like a filter, scattering red while allowing blue to pass. For maximum blue, the particles should all be very close to the same size--about a millionth of a meter across. Khamaseen storms are notorious for such fine dust. The spring dusty season is just beginning in Egypt. Sky watchers there should be alert for more blue suns--and blue moons, too.